Electroluminescent lamp



April 1961 E c PAYNE ELECTROLUMINESCENT LAMP 2 Sheets-Sheet 1 Filed Jan. 9, 1953 ELMER C. PAYNE INVENTOR.

BY 22 6 v ATTORNEY April 18, 1961 Q YNE 2,980,816

\ ELECTROLUMINESCENT LAMP Filed Jan. 9, 1953 2 Sheets-Sheet 2 FIG.6

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DONATOR GAPZ/ I9 ACT/VATOR MAIN ENERGY m L GAP I7 VALENCE BAND /6 FIG.7

REGION OF HIGH FIELD REGION OF HIGH FIELD REGION OF HIGH FIELD INVENTOR.

} ELMER C. PAYNE W 64km,

ATTORNEY United i 1 2,980,8l6 ELECTROLUMINESCENT LAMP Elmer Curry Payne, New Brunswick, NJ., assignor to Sylvania Electric Products Inc., Salem, Mass, a conporation of Massachusetts Filed Jan. 9, 1953, Ser. No. 330,543.

9 Claims. c1. sis-10811 This invention relates to lamps in which light is produced by the application of an electric field to a region including a phosphor. The invention relates also to phosphors particularly suited for such use, and to methods of preparing them.

Electric lamps utilizing phosphors have been previously known. In one commercial type, the electric field is applied to a gas and the resultant ultra-violet radiation used to excite the phosphor. applied to an evacuated space in which electrons are accelerated and the resultant cathode raybeam used to excite the phosphor. But in neither of these devices is the light produced by the direct application of a sufficiently intense field to the immediate region of the phoshor. 9' Moreover, both of these devices, like the well-known incandescent lamp, require an enclosing bulb or tube hermeticallysealed, with consequent difliculties in manufacture and limitations in geometry. The devices of my invention are far less limited in such respects, requiring no sealed bulb or tube for their operation, and can be made in practically any convenient size and shape, thus making possible for the first time such new lighting applications as the illumination of a room by a complete ceiling of electrically-luminous plaques.

As in certain specific embodiments of the invention described below, such plaques advantageously have a plate of conductive glass in contact with one side of a thin layer of phosphor embedded in a light-transmitting dielectric material, and a conducting layer in contact with the other side of said phosphor layer.

'The 'device has a positive volt-ampere characteristic and hence requires no ballast. It can be operated directly from the usual 110 volt, 60-cycle per second power line,

although in some cases a transformer may be desirable.

for higher voltage operation.

The device is in efiect a luminous capacitor, and the light produced appears to be due to the action of the electric field on the phosphor, or on the phosphor and the embedding material.

The appearance of a faint glow upon the application of an electric field to a phosphor film appears to have been observed heretofore, but only as an obscure scientific phenomenon. Attempts by earlier scientists to produce light of practical illuminating intensities by such means appear to have been unsuccessful, as pointed out by Leverenz in his recently-published book, An Introduction to the Luminescence of Solids (McMillan, New York, 1950, page 290). Leverenz found that excitation of allegedly field-excited phosphor films produced only a feeble blue glow, barely perceptible-to a partially darkadapt'ed eye, and he attributed the glow to excitation of aerospheres of atmospheric nitrogen trapped in the embedding medium, rather than to excitation of the phosphor itself by the field.

- .In contrastto this, the light produced by the present invention is not at all the blue glow of atmospheric nitrogen, and can be made blue, green, yellow, red or In another, the field is' other color depending on the choice of phosphor. And instead of the faintly perceptible glow of the films examined by Leverenz, my lamps can have brightness as high as 20-foot-lamberts or more. This brightness can be maintained over large areas to yield a very large total light output.

These unexpected results appear to be due to direct excitation of the phosphor by the field. I have discovered that useful electroluminescent phosphors generally comprise a host crystal containing not only an activator impurity, but also a donator impurity, the latter providing,

electrons, which'can be excited into the conduction band" of the crystal, with the expenditure of a small energy; and from there can be accelerated by the field until they have enough energy to excite the activator atoms. For example, one very effective electroluminescent phosphor comprises zinc sulfide with a copper activator impurity and a lead-donator impurity.

- Copper-activated zinc sulfide containing lead has been known as an infra-red responsive phosphor, but the amount of lead used for such infra-red phosphors is considerably more than can be used in an eificient electroluminescent phosphor. The amount of retained lead in infra-red phosphors is greater than the highest amount which gives appreciable electroluminescence. An infrared phosphor does not ordinarily electroluminesce, and an electroluminescent give infra-red response.

Once an electron is freed from a donator atom in the crystal, it is accelerated through a considerable number of atom spacings by the field. For example, with 600 volts across three-thousandths of an inch layer of phosphor, the two-volt drop necessary for visible emission may require a distance of about 2.5 10- centimeters, or about-2000 atom-spacings. The mean free path in crystals with loosely-packed atoms, such as zinc sulfide crystals, under the influence of a field, may be itself as great as 10* or more, so that the meanfree path and the required distance of travel may not be greatly different. However, the electron can acquire sufficient energy by travelling for a considerable number of mean free paths, if the energy losses at the end of each path are small. Loosely packed crystals such as the sulfides, sele nides, and silicates of cadmium, zinc, and the like, have;

large mean free paths and are thus suitable for electroluminescence, when a suitable activator and donatorv arepresent.

When the field is sufficiently high, it may producefree electrons from atoms, by shifting electrons directly from a lower band to the conductive band, and the presence of some electroluminescence may be obtained under such circumstances without a donator impurity, but the addition of a donator impurity will greatly increase the light emission even in such cases. For example, in one copper-activated, lead donator, zinc sulfide phosphor, the light emission was increased from a barely visible illumination of 3 units to a bright illumination of 460 units the space between two electrically conducting layers, at least one of which is light-transmitting suchas a conduct ing glass or plastic. In the absence of the embedding. material, the crystals appear to glow only at their points of contact with a conductor, that is with each other or with an electrode. The presence of the embedding medium greatly enhances the glow, which can be seen under,

the microscope to spread out over a larger part of the entire crystal, although the luminescence is even then con-Q Patented Apr. 18, 19.61

phosphor does not ordinarily fined to spots on the crystal in some cases. The phosphor of zinc oxide aflixed to the main zinc sulphide crystals, or

they may be of lead sulfide or copper sulfide, or they may be due to some other distorting influence.

A certain amount of zinc oxide is helpful in the zinc sulphide mixture while the latter is fired, because it aids in formation of the proper type and constitution of crystals. Nonetheless washing the phosphor with asuitable solvent for zinc oxide, such as a solution of acetic acid or of ammonium acetate, improves and multiplies the brightness of such phosphors. Such solvents also have the effect of removing surface impurities from the surfaces of the active crystals.

The combination of the heat treatment during firing and the subsequent chemical treatment produces a depletion layer on the surface of the crystal particle, that is a layer from which some or all of the donor material is absent. Thus when two particles are joined together to form an aggregate, there is an insulating space between them comprising the depletion layers of each crystal, with the main internal semi-conducting body of each crystal: acting as electrodes on each side of the depletion layer, with a consequent high. field in the layer.

The same sort of effect can occur when there is a conducting particle, such as a metal or a semi-conducting particle, like lead or copper sulfide, or copper oxide, on the surface of the crystal near or over the depletion layer, and the presence of such a metal or semi-conducting particle can produce an exhaustion layer or a depletion layer by the chemistry of its own formation.

Sharp points or projections on the crystal will also provide a high field in a depletion layer over them, and sharp points or projections on a metal or semi-conducting particle on the surface of the crystal will also increase the field in the surrounding portion of the main body of the crystal.

Although the concentration of donator material in the depletion layer is less than that in the main body of the crystal, the concentration of activator centers is generally sufiicient to allow the emission of visible or other desirable radiation or light, either by direct excitation of the activator centers by the field, or by the acceleration of electrons to sufficient energy for excitation. Where the field in the depletion layer is high enough to extract electrons directly from the metal or semi-conductor at one end of the depletion layer, it is not necessary to have activator atoms in said layer, as the electrons can be accelerated to high energy and then excite the activator atoms at the joints of the depletion layer and the main semiconducting body of the crystal. At lower fields, and in other cases where the electrons get into the depletion layer from the main semi-conducting body of the crystal, however, there should be some activator centers in the depletion layer itself.

The presence of activator atoms in the depletion layer itselfis shown by the fact that with the acid treatment described above, the zinc oxide and lead present in the phosphor are considerably reduced, but the copper content is not greatly changed.

The emission from the sulphide phosphors is thus due to a surface layer between the dielectric medium and the main portion of the crystal. This depletion layer, being when the resistivity of the phosphor in a powder is meas-= of an insulating nature, increases the phosphor resistivity e saw.

ured. The resistivity of the main semi-conducting body of the crystal has a smaller value. The main semiconducting body of the crystal may itself consist of several different types of material, some of which may be insulators, thus further increasing the gross measured resistivity, especially if the measurements are made on DC Other features and advantages of the invention will be apparent from the following detailed description of specific embodiments thereof.

In the accompanying drawings, three devices embodying aspects of the invention are shown, Fig. 1 being a perspective view partly in section of one such device; Fig. 2 being a perspective view of a second device; Fig. 3 being an enlarged cross-sectional view of the device in Fig. 2; Fig. 4 being a plan cross-sectional view of a third device along the line 4-4 in Fig. 5; Fig. 5 being an enlarged cross-sectional view of the device in Fig. 4, along the line 55; Fig. 6 being an energy diagram of a phosphor according to the invention; and Fig. 7 is a schematic enlarged section through a phosphor particle, showing the depletion layer.

The device shown in Fig. 1 has a glass plate 1, having a transparent conductive surface 2, over which is a thin layer 3 of phosphor-impregnated dielectric material, with a metal backing layer 4 over that and in intimate contact therewith. This completes an illumination source, suitable for use as a luminous plaque for walls and ceilings, for example. One terminal of a proper source of varying or alternating voltage can be connected to the metal backing layer 4, the other to a metal tab 5' which is connected to the conducting surface 2.

In a modification of this device the layer 4 can also be of conductive glass, instead of being of metal, thus providing a plaque which emits light from both sides, and when not energized in translucent. Such a device can be used in various ways, for example in table lamps and other lighting fixtures or even as a window pane which transmits sunlight by day and emits its own light at night.

A conducting surface 2. of good transparency or translucency is difiicult to obtain, because good electric conductors are generally good reflectors of light, rather than transmitters of it. However, although other coating may be used, I find that a particularly effective conductive surface may be provided by heating the glass and exposing it while hot to vapors of the chlorides of silicon, tin, or titanium, and afterward placing the treated glass in a slightly reducing atmosphere. Where the application in the vapor state isnot convenient, good results may be obtained by mixing stannic chloride with absolute alcohol and glacial acetic acid and dipping the glass plate into the mixture.

However applied, the resultant conductive surface ap pears to contain stannic (or silicic or titanic) oxide, probably to some extent at least reduced to a form lower than .the dioxide, although the exact composition is difficult to determine.

The conductive surface 2. so applied will have a resistance of about 100 ohms per square, that is a resistance of 100 ohms taken between the entire opposite sides of any square on the surface 2.

The phosphor-impregnated layer 3 placed over the transparent conductive layer 2 is a phosphor of copperactivated zinc sulphide as described below, in the form of fine particles embedded in plasticized nitro-cellulose.

For example, about 20 grams of nitrocellulose with a suitable plasticizer such. asv chlorinated diphenyl can be dissolved in about cc. of of a suitable solvent such as butyl acetate, and about; 10 grams of finely-divided phosphor suspended therein. The plasticizer-in the above example is included in the weight of the nitrocellulose, which can be of quarter-second viscosity, although any convenient viscosity can be used, the proportions and constituents of the above mixture being capable of considerable variation. A large number of plasticizers are: well known, but for best results those with high'resistivity and high dielectric constant should be chosen. The plasticizer may be used in considerable quantity, if desired, and may even comprise the major portion of the combined weight of nitrocellulose and plasticizer, as shown in the application of Eric L. Mager filed concurrently herewith, and a non-acid dielectric medium, or one of low acidity or acid number may be used as shown in that application.

The backing layer 4 is of metal, preferably a good reflecting metal such as aluminum or chromium, which will not react appreciably with the phosphor or embedding material used; The metal layer or conductive surface 4 is preferably of low resistance and;can be applied in .any convenient manner, taking care not to damage the cellulose-phosphor layer. However, best results have been obtained by vacuum-deposition of the metal. The glass platel, with itsconductives urface 2, is coated with the embedded'phosphor layer 3, placed in a bell jar and the latter evacuated. The coating 3 is then heated'for 'a" moment, for example by passing a current through the conductive'surface 2. The heating is preferably of the order of that used for drying, and should not, of course,

be sufiicient'to char the embedding material in phosphor layer 3. The heating is not essential to producing a plaque of good initial brightness, but aids in maintaining the brightness throughout the life of the lamp.

The aluminum or other metal is then deposited on the phosphor layer in a vacuum, for example by being placed on a tungsten filament and'the latter heated by the passage of an electric current therethrough, as shown for example in U.S. Patent 2,123,706, issued July 12, 1938, to O. H. Biggs.

' Various other plastics can be used instead of nitro-' cellulose. Glass and various enamels may be used, particularly glass 'of low enough melting point to insure thatthe phosphor crystals remain unmelted.

The thickness of the various layers can-be altered to suit various voltage conditions and the like. The voltage necessarily will depend on the phosphor used, the thickness of the phosphor layer 3, and the brightness desired, but voltages between 25 volts and 2500 volts and even higher havebeen used; A lamp operable from'a 110- volt alternating current power line can be made with the conducting surface 2 of a thickness of about a wavelength of light, producing an iridescent effect when viewed at an angle, the phosphor layer'3 ofabout 2 one thousandths of an inch, and the metal layer 4 of afraction of a thousandth of an inch. The plate 1 can have any convenientthickness and should be transparent or translucent. --A- highly effective phosphorcan be prepared by intimately mixing as fine powders about 75 parts .by weight of zinc sulphideand 25% parts zinc oxide,.with about 1.0 part zinc chloride, about .75 part copper added as copper-sulphate, and about 1 part lead sulphate.

These are the preferred. values for best results, but the copper, calculated as metallic copper, can be varied over a range of about 0.03% to 0.3%, and the amount of chloride-calculated as zinc chloride, should be between 0.4% .to 2.0%, although if a fluoride is used the amount added should .not ordinarily be greater thanabout- 0. 1%; -Theamount of lead, calculated as the sulphate, should be. between 4% and 5%, the. higher amounts being used only when there is a considerable flow of nitrogen .or otherinertgas duringthe firing, to carryaway the excess lead. The amount of lead retained in the final phosphor should he only between about 0.01% and 0.000l% by .Weight for best brighte'components should be thoroughly mixed in the form of fine powders, andheated to between 900 to, 1250 C. in an inert atmosphere, for example in a gastight-electric furnace filledwith nitrogen, and having a chamber of nitrogen connected thereto to pass a .slow. stream ofthe gas therethrough. A batch of 200 grams has/been fired in an electric furnace at about 10009.,

C. 'in a small silica boat in a silica tube 3 inches in diameter and 30 inches long sealed at both ends, with.

a quarter inch silica tube feeding nitrogen into one end of the tube and a quarter inch tube for exhausting the nitrogen at the other end. The rate of flow" ofnitrogen was about 0.1 liter per minute.

In firing the phosphor, there are three distinct stages.

In the first stage, the mixture emits fumes of the halide and the lead compound used, and turns a yellow color,.

The soft, fiuffy mass can then be crumbled or shaken.

to separate the powder particles.

After firing of the phosphor, a treatment with acetic acid or ammonium acetate improves the luminescence" of the phosphor. The brightness is usually increased several times, and in many cases the treatment makes the diiference between a good brightness and no brightness atl all.

In treating the fired phosphor powder with acetic acid, 5

a solution of about 5% of the acid in water is heated to between 60 C. and 100 C., preferably nearer to.

60 C and poured over the phosphor while the latter is subjected to a gentle grinding action until thoroughly treated, for example, about 2 minutes, and the suspension is then filtered, washed with water, and dried. The

temperature of the solution is kept at about 60 C. to' 100 C. during the entire treatment, even during the.

filtering.

While the foregoing treatment improves the phosphor, an ammonium acetate treatment may be preferred because it'is less critical in use and is more effective in increasing the brightness. In some cases, the acetate has given a brighter phosphor than the acetic acid. In using this treatment, enough of a saturated solution of ammonium acetate in water is added to the phosphor to give a slurry, which is stirred in a mortar and thor-' oughly ground until all the large aggregates are broken up into their component particles. Then a quantity-of half-saturated acetate solution is added, in proportions of say 200 cc. for every grams of phosphor, enough to give a thinner slurry and the supernatant suspension poured off. A similar amount of half-saturated acetate;

solution is then added to the phosphor and poured off or filtered off. The process is repeated with successive dilutions, two treatments with one-eighth saturated solution, then two with one-twelfth saturation, two with onesixteenth and several with pure water. The treatment could, if desired, be continuous with two streams of liquid, one being of water and one of acetate solution, pouring onto the phosphor, the stream of acetate solution being gradually reduced in flow. If the acetate concentration on the phosphor is not diluted gradually, the dissolved zinc oxide will precipitate out again over the.

phosphor.

The elfectiveness of the treatment is apparently in removing the excess zinc oxide, leaving the sulphide and presumably leaving also any small particles of zinc oxide which may have attached themselves to the sulphide, or. any zinc oxide distributed throughout the sulphide crystals.

The treatment is also found to remove a considerable fraction of the lead.

If the acetic acid solution is used, it should be enough to remove the oxide Without also removing the; sulphide. With the acetate solution, the sulphide is"un=-" afiected, and the pH of the solution may be varied from 4 to 9, by varying the proportions of the ammonium and acetate radicals, and still be effective. This is a helpful circumstance, for commercial ammonium acetates generally vary in composition, differing considerably from stoichiometric. The acetate solution may possibly remove also any surface film on the sulphide crystals and perhaps some of the copper, although the latter seems less likely.

Other ammonium salts, such as the chloride, are also effective, but ammonia itself is not satisfactory. The reason appears to be that in a reaction between zinc oxide and ammonium acetate a complex zinc diarnine acetate is formed, plus water. Ammonia itself lacks a negative radical to form such a complex for removing both the oxide and Zinc portion of the Zinc oxide. But

ammonium chloride and many other ammonium salts would work like the acetate.

The effects of the treatments described on a ZnS-ZnO phosphor, suitably activated, are indicated in the following table:

Relative Brightness Percent ZnS In Starting Mixture Untreated Treated The treatment in the above tests was with acetic acid. The ammonium acetate treatment generally gives about 50% more brightness near the maximum point.

It is seen from the table that the treatment had no effect on the 100% zinc sulphide sample, presumably because there was in that case no zinc oxide to be removed, but in the other cases the treatment had a remarkable elfect in increasing the brightness. The untreated samples appeared to have no appreciable luminosity before treatment.

The test device actually passed 100 times as much current with untreated phosphors as with treated ones. With a sinusoidal 100 volts at 60 cycles per second on a test device using a cell 0.01 inch thick and 5 sq. in. in

area, between a metal plate at the bottom and a piece of conductive glass at its top, using 1.5 grams of phosphor in 1.2 cc. of castor oil, the current passed'was 5 milliamperes before treatment and 0.05 milliampere after treatment. Thus a high-resistivity phosphor is defined for the purposes of this specification as one which passes of the order of 0.05 milliampere when tested in the above apparatus under the conditions specified, that is one which does not pass more than 0.5 milliampere.

The resistivity of the treated phosphor is even greater than the current passed through the cell might at first seem to indicate, because a large part of the current through the cell is due to capacitance. The current of 0.05 milliampere when the cell is filled with the castor oil and treated phosphor inthe above example is about double that passed when the cell is filled with castor oil alone. Since the phase angle between voltage and current is only about five or ten degrees, only a small part of the increase in current is due to the conductivity of the treated phosphor. The greater increase appears to be due to the increased dielectric constant of the mixture with the treated phosphor added. Since the dielectric constant of the phosphor-oil mixture appears to be double that of the oil alone, the constant of the treated phosphor appears to be quite high, say greater than ten, because the phosphor makes up only about a third of the volume of the mixture in,.the cell: 7

But when thephosphor is untreated, there is a consider- S able amount of zinc oxide present, and since the dielectric constant for the oxide appears to be only about 2.5, the great increase in current with the untreated phosphor is due' toits high conductivity.

A "further example of a phosphor useful in electroluminescent devices has been prepared by intimately mixing the following ingredients as finely-divided powders, in the proportions indicated:

' Grams Zinc sulphide (Zns) 75.60 Zinc oxide (ZnO) 24.42 Lead carbonate (PbCO 1.87 Cuprous oxide (CuO) 0.0637

The sulphide used contained 5 grams of water and 1% zinc chloride. The water was removed by drying the mixture at 160 C. The batch was then placed in a quart mill and milled with acetone for half an hour, after which it was again dried and then fired at 1000 C. in an open silica tray, for half an hour in a furnace. Pre-purified nitrogen was flowed over the mixture in the furnace at a rate of 0.06 liter per minute, during the firing. After this heating, thephosphor was ground lightly in a mortar to break up the. resultant fiulfy cake into its component particles or into its smalleraggregates of particles.

The powder was then treated with a boiling solution of 5% acetic acid in water, then with a /2% solution of the same and then washed with water. In each case the phosphor was placed in the acetic acid solution and the whole raised to boiling temperature in about ten minutes, continuing the boiling for five minutes.

In phosphors using about 25% oxide in the starting mixture, the percentage has been found to be still substantially 25% after firing. But after treatment with the solutions as above described, the amount of oxide present has been reduced to 5% or less, so that the final phosphor is about sulphide or more.

Phosphors made without any oxide in the starting materials have luminesced but were only about 20% as bright as those made with oxide. The copper content for even the 20% brightness should be much greater than the optimum values for the phosphors using oxides. A small amount of oxide may possible be formed in this phosphor during, firing, since it is difiicult to insure that every trace of oxygen is absent from the nitrogen atmosphere used during firing.

' The sulphide phosphor prepared as in the preceding examplesfluoresces a greenish-yellow under excitation at 60 cycles per second and fluoresces blue under excitation of about 2000 cycles.

A phosphor fluorescing orange-yellow may be prepared by using manganese in activating amounts with the sulphide. In an example, such a phosphor is produced by mixing 87 parts by weight of zinc sulphide and 13 parts zinc oxideas finely divided powders, together with about 2.1%.manganous sulfate and 0.8% zinc chloride and 1% lead sulfate. The constituents are blended as dry powders, preferably finely-divided, or they may be blended wet, for example, in a water slurry and dried. Some of the compounds will remain as powders and some will dissolve in the water.

In any case, after the components are thoroughly mixed together, the resultant mixture should be fired, preferably in an inert atmosphere, as with the previouslydescribed phosphor, at a temperature of about 900 C. to 1200 C., preferably about 1000 C. After firing, the resultant mass is crumbled or milled to a desired particle size, although the less the milling, the better will be the phosphor.

The phosphor should then be given a treatment such as the acetic acid or ammonium acetate treatment previously described, toimprove its brightness. Such a treatment appears to be less marked in its efiect on the Among the other manganese-activated phosphors which exhibit electroluminescence are the cadmium silicate and zinc fluoride phosphors. Electroluminescent sulphide phosphors can also be made in which the zinc is replaced partly or entirely by calcium or strontium.

.Figs;. 2;to' 5 show forms of the invention in which paired "long spaced narrow conductors 6 and 7, 8 and 9, are placed side by side, the conductors and the space between them being occupied by a coating or layer 10, 11 consisting of an electroluminescent phosphor embedded in a dielectric material. The conductors and the layer are carried by insulating supports 12 and 13. In Fig. 2 the conductors 6, 7 are wires, having an enamel insulating layer 14, wound side by side and close together but spaced apart a distance of a few thousandths of an inch or less.

A lamp is defined for the purposes of this specification as a device which produces light of practical illuminating intensities. Intensities below a foot-lambert are practical for some application, although the lamps herein described have given several foot-lamberts on 60 cycles per second alternating voltage supply, and 15 to 20 footlamberts on a supply of several thousand cycles per second.

Such lamps are therefore useful for general illumination purposes including use as luminous panels for ceilings, as lighting sources for table lamps, as luminous signs and clock faces, as luminous face plates for household electrical switches, for street lighting and for many other applications.

A typical energy level diagram for an electroluminescent phosphor is shown in Fig. 6. Between the highest filled band 15 of the host crystal and the lowest unfilled band 16, there is an energy gap 17. If the phosphor is to emit visible light, the width of this gap must be greater than 1.5 electron volts, which corresponds to the energy equivalent to the extreme red end of the visible spectrum, that is to the lowest energy which can produce visible light. In zinc sulfide the gap is about 3.5 volts.

The highest occupied band 18 of the activator material will be spaced from the lowest unfilled band 16 by an amount less than for the gap 17, and in the case of copper in zinc sulfide will be spaced about 2.7 volts. The

donator impurity will have its highest occupied level 19 just below the bottom of the lowest unfilled band 16, and the gap between the two will generally be only a few tenths of a volt, for example between about 0.1 volt and 0.4 volt for a lead impurity in copper-activated zinc sulfide. Other activating materials can be used, for example silver or manganese, and the most suitable material will be different for different host crystals. A host material of fairly open crystal structure and consequent long mean free path facilitates the acceleration of the electrons to a value suitable for exciting the activator material. Metallic sulphides, selenides and silicates are among the compounds which can be used as host crystals. Zinc, cadmium, and calcium are among the substances which can be used as the metallic component of the compounds.

A donator impurity is one from which electrons can be freed by thermal agitation or by the field at relatively low energy values, generally not over a few-tenths of a volt. A donator impurity is a substance having a higher valence than one of the atoms or ions of the host crystal so that it can substitute for such an atom or ion, and lose an electron in the process. The lost electron will take up a low energy orbit of large radius around the atom or ion from which it has thus been partly freed, and may then be more nearly completely freed by a small additional energy, the amount supplied by thermal agitation often being sufficient. For example, lead, thallium, indium, and tin are among the metals which can replace 10 part of the zinc 'in zinc 'sulphide'and act as donator impurities.

For best brightness, the amount of donator material,

present should be less than about 0.00005 gram-atom per mole of host crystal material, although with some phosphors electroluminescence can be obtained with two 'to three times that proportion of donator materialwThe' amount of donator material should, of course, be greater than zero and for best brightness should generally be above 0.0000005 gram-atom per mole of host crystal material.

When chlorine is used in the starting mixture from which the phosphor is made, the amount retained in the final washed phosphor will not ordinarily be greater than 0.1% by weight of the final washed phosphor.

activators of the types held in part at least interstitially in the phosphor are particularly effective, and the amount of such activators used is much greater than theamount ordinarily used in cathode ray or ultraviolet excited phosphors, or in infra red-responsive phosphors, as shown by the comparatively high copper content of the electroluminescent zinc sulphide phosphor described above.

The highest occupied band 19 is generally known as the valence band and the lowest unfilled band 16 as the conduction band.

This application is in part a continuation of my copendrng applications Serial Nos. 105,803, now abandoned, 119,021, now abandoned, 119,022, now abandoned, 180,- 783, now Patent No. 2,838,715, and 288,603, filed respectlvely July 20, 1949, September 30, 1949, September 30, 1949, August 22, 1950, and May 19, 1952.

In Figure 7, two phosphor particles are shown, having their main body of a semi-conducting material 21, 22, covered by a depletion layer 23, 24. In the case of a zinc sulphide crystal, for example, the semi-conducting part of the crystal could be ZnSzCu, Pb and the depletion layer ZnSzCu.

When the crystal is embedded in a dielectric medium, there will be a high field in the depletion layer around projections like 25, for example, and also in the depletion layers 23, 24, between the two semi-conducting portions 21 and 22, where the two particles are joined together, to form a conglomerate. There will also be a high field in the depletion layer 23 between the conducting spot 26 on the outside of the crystal, and the internal semiconducting portion 21.

Although the interior of the crystal is shown as being a uniform semi-conductor, it may actually be composed of several diiferent regions of variable properties, and those differences may provide additional spots where the field is high.

What I claim is:

1. An electroluminescent phosphor comprising particles of a semi-conductor material having a surface depletion layer.

2. An electroluminescent phosphor comprising particles of a semi-conductive material having an activated depletion layer.

3. An electroluminescent phosphor comprising a particle of a semi-conductive material having a depletion layer in contact with a conducting material.

4. An electroluminescent lamp comprising a particle of a semi-conductor material having a projection, a depletion layer over said projection and a dielectric material in contact with said depletion layer.

5. An electroluminescent phosphor comprising a particle of a semi-conductor material, a depletion layer at the surface of said particle, and a semi-conductor material over at least part of said depletion layer.

6. An electroluminescent phosphor comprising a particle of a semi-conductor material, a depletion layer at the surface of said particle, and a conducting material over part of said depletion layer.

7. An electroluminescent phosphor comprising parti- 11 cles. of a semi-conductor material containing donator material and having a layer depleted of said donator material but containing activator centers.

8. An electroluminescent phosphor comprising particles of a semi-conductor material containing donator material distributed in such a manner that the particles have an outside layer with a donator concentration smaller than that in the main body of the crystal, said layer also containing an activatormaterial.

"9i The phosphor of claim 8', in. which the semi-com ductor material is zinc sulfide, the activated copper, and the donator material thallium.

References Cited in the file ofthi'slpatent UNITED STATES PATENTS" 

